On 16 Apr 2006 15:44:06 -0700, "K7ITM" wrote:

OK, I gotta take issue with the part that says,

" A transmission line, even a very good one, generally has a Q of
someplace around 20-75. The definition of Q I am using is reactance
over ESR. Say you need a reactance of 400 ohms to resonate an antenna.
Linear or stub loading would add a series resistance of 5 to 20 ohms as
loss resistance at that point in the system.
"

I know that transmission line Q varies all over the place: it's much
more reasonable to use it in a resonator at high frequencies than low,
and line construction makes a big difference too. To back this up with
numbers, I just ran some calcs (actually put together a little Scilab
program to run them for me) on four different lines: (a) is RG-8/RG-213
type line with solid poly dielectric, (b) is 75 ohm air insulated coax
in an 0.5" ID copper tube, (c) is balanced two-wire line made with
12AWG (~2mm) wire spaced 2" (~5cm) on centers), and (d) is two 0.625"
OD copper tubes spaced 3" on center.

For a 1/8 wave section of line shorted at the far end, the calculated
impedances and Qs a

line a, 10MHz: 0.622+j50, Q=80
line a, 100MHz: 0.197+j50, Q=254
line a, 1000MHz: 0.0622+j50, Q=800


I tried these numbers in the line loss calculator at
http://www.vk1od.net/tl/tllce.php using Belden 8267 of 2.475m length
for 0.125 wavelengths and Zload=0.0000000001. The input Z I got was a
little higher at 0.88+j50 (probably slightly different approximation
of Zo used in the calcs), yeilding a Q of 57. The Q is quite dependent
on line length, decreasing as length increases towards a quarter wave.

I suspect this is not a good method of analysing behaviour when the
line elements are field coupled to other radiator elements, the
currents in each leg are not necessarily equal and opposite.

Owen
--

Yes, the Q as determined by simply taking X/R decreases as you approach
1/4 wavelength, but what you really need to do is resonate it with a
capacitance and look at the Z as a function of frequency when you do
that. I mean, it IS a resonator if it's 1/4 wave long: it would look
like Q=0 there if you take X/R, but of course it's not. If you simply
want _inductance_ (i.e. a loading coil), do NOT make the stub close to
1/4 wave long. It's just the same as trying to use a coil for
inductance up near its self-resonance.

Also, a point that was in my mind when I originally posted, but failed
to put well into writing then, is that as frequency increases, the Q of
a solenoid coil will increase about as the square root of
frequency...and the size stays the same. But the stub's Q also
increases as the square root of frequency, while it's size (length) is
directly proportional to 1/freq, and it's shrinking in size.

And thanks for the cross-check on my numbers, Owen. I hacked it pretty
quickly, and may have missed a cog somewhere, though I think the
numbers are reasonably close. I suppose one of Reg's programs will
give you stub impedance, too. -- I think I see why my numbers may be
a bit different than what you got; I'll check on it as I have time,
though the difference isn't enough to worry me--the trends are still
the same.

Cheers,
Tom

Owen Duffy wrote:
On 16 Apr 2006 15:44:06 -0700, "K7ITM" wrote:

OK, I gotta take issue with the part that says,

" A transmission line, even a very good one, generally has a Q of
someplace around 20-75. The definition of Q I am using is reactance
over ESR. Say you need a reactance of 400 ohms to resonate an antenna.
Linear or stub loading would add a series resistance of 5 to 20 ohms as
loss resistance at that point in the system.
"

I know that transmission line Q varies all over the place: it's much
more reasonable to use it in a resonator at high frequencies than low,
and line construction makes a big difference too. To back this up with
numbers, I just ran some calcs (actually put together a little Scilab
program to run them for me) on four different lines: (a) is RG-8/RG-213
type line with solid poly dielectric, (b) is 75 ohm air insulated coax
in an 0.5" ID copper tube, (c) is balanced two-wire line made with
12AWG (~2mm) wire spaced 2" (~5cm) on centers), and (d) is two 0.625"
OD copper tubes spaced 3" on center.

For a 1/8 wave section of line shorted at the far end, the calculated
impedances and Qs a

line a, 10MHz: 0.622+j50, Q=80
line a, 100MHz: 0.197+j50, Q=254
line a, 1000MHz: 0.0622+j50, Q=800


I tried these numbers in the line loss calculator at
http://www.vk1od.net/tl/tllce.php using Belden 8267 of 2.475m length
for 0.125 wavelengths and Zload=0.0000000001. The input Z I got was a
little higher at 0.88+j50 (probably slightly different approximation
of Zo used in the calcs), yeilding a Q of 57. The Q is quite dependent
on line length, decreasing as length increases towards a quarter wave.

I suspect this is not a good method of analysing behaviour when the
line elements are field coupled to other radiator elements, the
currents in each leg are not necessarily equal and opposite.

Owen
--

You dim witts are calculating Q incorrectly.

Q = X / R

where R is the RF resistance of the conductor and X is the reactance
of the conductor's inductance. You first have to calculate
inductance.

You get a high Q at resonance.
----
Reg, G4FGQ

On Mon, 17 Apr 2006 08:41:36 +0100, "Reg Edwards"
wrote:

You dim witts are calculating Q incorrectly.


Reg, that is just so polite!

Q = X / R

where R is the RF resistance of the conductor and X is the reactance
of the conductor's inductance. You first have to calculate
inductance.


So, you state that the ratio X/R is an acceptable way to express the Q
of an inductor, why is it unacceptable to express the Q of a two
terminal device with an equivalent series impedance of 0.88+j50 (where
0.88 is the RF series resistance of the network and 50 is the series
inductive reactance of the element) as 50/0.88 or 57?

Aren't the effiency implications (for that was the context) for a 50
ohm reactance created with a TL stub as described just the same as for
a coil with 50 ohms of inductive reactance and 0.88 ohms of series
(RF) resistance, ie a coil with the same Q factor?

Owen
--

"Owen Duffy" wrote in message
...
On Mon, 17 Apr 2006 08:41:36 +0100, "Reg Edwards"
wrote:

You dim witts are calculating Q incorrectly.


Reg, that is just so polite!

Q = X / R

where R is the RF resistance of the conductor and X is the
reactance
of the conductor's inductance. You first have to calculate
inductance.


So, you state that the ratio X/R is an acceptable way to express the
Q
of an inductor, why is it unacceptable to express the Q of a two
terminal device with an equivalent series impedance of 0.88+j50
(where
0.88 is the RF series resistance of the network and 50 is the series
inductive reactance of the element) as 50/0.88 or 57?

Aren't the effiency implications (for that was the context) for a 50
ohm reactance created with a TL stub as described just the same as
for
a coil with 50 ohms of inductive reactance and 0.88 ohms of series
(RF) resistance, ie a coil with the same Q factor?

Owen
----------------------------------------------------------------------
---
Owen,
Please excuse my mild scold.

There is only ONE way to calculate Q of a coil or a wire and that is
the way I have described.

It is the ratio of inductive reactance to resistance of the wire, in
series with other. They cannot be measured in combination with each
other.

To do so results in something altogether different like measuring the
input impedance of an antenna at or near resonance where the inductive
reactance is tuned out by the capacitance and is therefore NOT
measured.

It is elementary my dear Watson.
----
Reg.

Just an afterthought.

Q is dimensionless quantity. Therefore it cannot be measured directly.

It is always obtained as the CALCULATED ratio of TWO independent
measurements or previous calculations.

Its only use is to predict, by further calculation, other properties
of a circuit such as bandwith or voltage magnification. It is just a
convenient intermediary which can frequently be bypassed or done
without.

It can seldom be determined accurately which is a measure of its true
worth. Your guess is as good as mine at high frequencies.

The common or garden Q meter indicates only the resistance of a coil
relative to a standard of some sort. The coil's inductive reactance
is already known, or is related to the capacitor and frequency, or can
otherwise be calculated. Here you still have a pair of independent
measurable quantities.

I'd better stop here. The subject has been over-complicated quite
enough. Here in the Black Country, the weather is beautifully fresh.
Spring is well on its way.
----
Reg.

That (your afterthought) is much more like it. Thanks.

After all, this is NOT a thread about Q, it's a thread about the
effectiveness of different two-terminal devices for use in inductively
loading a linear radiator. In that case, the measured impedance, that
is, the measured X and R, of the two-terminal device is indeed what
matters. Given that we need a particular X, a high ratio of measured X
to measured R is advantageous, since the R term represents
dissipation. Maybe we should invent a new term and define it thus:

Xiddle = X(measured)/R(measured)

where Xiddle is to be pronounced "Ziddle," and rhymes with "piddle."
Or, we could just use the shorthand that W8JI elected to use AND DEFINE
in his posting: Q=X(meas)/R(meas).

Just as you say, Q is only an intermediate on the path to something
more interesting. It works for me if someone wants to offer a slightly
non-standard definition, so long as the definition is clear, as it was
to me from W8JI's post.

Thanks for mentioning the Black Country. It was an education for me to
look it up. Spring is trying to gain a toehold here, but it's a bit
tenuous. Got up to a couple feet of new snow in the hills over the
weekend.

Cheers,
Tom

(PS--where do you find gardens that grow "Q meters"? Or are they the
things that invade the garden to try to eat the qms?)

Reg, old sot, you need to either quit drinking so much or go back to
the fundamental definition of Q. The Q of a coil or capacitor is
always an abstraction from the definition. In any event, trying to use
a coil or a stub at or near its self (anti)resonance to get an
inductive reactance is a really bad plan. You can't separate the
self-capacitance from the coil or the stub, so assuming only the
reactance from the wire's inductance does you no good at all in that
case. As a resonator, it's fine. As an inductive loading component
(which is the topic of this thread), it sucks.

Cheers,
Tom